M. Sharma

Growth of micropipe-free single crystal silicon carbide (SiC) ingots via physical vapor transport (PVT)

The move towards commercialization of SiC based devices places increasing demands on the quality of the substrate material. While the industry has steadily decreased the micropipe (MP) levels in commercial SiC substrates over the past years, the achievement of wafers that are entirely free of MPs marks an important milestone in commercialization of SiC based devices. We present the results of a study for controlling the nucleation and propagation of MP defects in SiC ingots grown via PVT. Our studies confirm that during bulk growth of SiC, foreign polytype nucleation such as 3C-polytype occurs at the initial stages of growth (nucleation period) and/or during subsequent growth in the presence of facets. Results in this investigation suggest that polytype instability during crystal growth adversely impacts the MP density. Based on this key concept, growth conditions for nucleation and growth stages were optimized. These conditions were subsequently implemented in an innovative PVT growth environment to achieve a growth technique with highly effective polytype control. Under continuously modulated growth conditions, MPs induced by seed material and/or formed during the growth were eliminated. 2-inch and 3-inch diameter MP-free (zero MP density) conducting 4H-SiC ingots were obtained.

TEMPERATURE-DEPENDENT STUDIES IN SOME HOMOLOGUES OF ALKYLOXY CYANOBIPHENYL EMPLOYING POSITRON LIFETIME SPECTROSCOPY

Some homologues of alkyloxy cyanobipheny1 (6OCB, 8OCB, 9OCB, 10OCB) have been investigated by employing positron lifetime spectroscopy. In each of these compounds, temperature dependent positron lifetime measurements have been carried out, both in the heating as well as cooling cycles. Besides detecting many interesting features like solid-crystalline polymorphism, anti-parallel bimolecular association, formation of cybotactic groups in the nematic phase, positron annihilation parameters have been able to reveal anomalous structural changes taking place in these compounds.

STUDY OF MOLECULAR MOTIONS IN TWO LIQUID CRYSTAL FORMING COMPOUNDS EMPLOYING PLS

Results of molecular motion studies carried out in two liquid crystal forming compounds n-p-cyano-p-hexyloxybiphenyl (M18) and n-p-ethoxybenzylidene-p-butylaniline (EBBA) using positron lifetime spectroscopy (PLS) are presented. Temperature dependent positron lifetime measurements have been performed in each compound during the heating cycle of samples prepared by either quenching or slow cooling from the respective liquid crystalline phase of the compounds. In both the compounds, behaviors of the quenched and slow cooled samples are found to be different. The material in the quenched sample, unlike the slow-cooled sample, exhibits strong temperature dependence before it undergoes a glass transition. In each case, the temperature dependence of o-Ps pick-off lifetime in the quenched sample shows broad peaks at various characteristic temperatures. These peaks have been attributed to various intra- and inter-molecular motions associated with these compounds. The characteristic frequencies of some of the modes observed in the present work agree well with the literature reported values obtained from FIR and Raman studies. The present study demonstrates the usefulness of PLS in the study of molecular motions.

Positron Lifetime Spectroscopy as a Tool for Studying Molecular Motions in n-p-Ethoxybenzylidene-p-Butylaniline

Temperature dependent positron lifetime measurements have been carried out in n-p-ethoxybenzylidene-p-butylaniline. The samples were prepared either by quenching or slow cooling from the liquid crystalline phase of the compound. The lifetime measurements were performed in the heating cycle of these samples in the temperature range 165 K to 355 K. The behaviour of the quenched sample is found to be quite different from that of the slow-cooled sample. The temperature dependence of o-Ps pick-off lifetime in the quenched sample, unlike the slow-cooled sample, exhibits eight distinct peaks at various characteristic temperatures before the material undergoes a glass transition. These peaks have been attributed to various intraand intermolecular modes executed by the molecules. The characteristic frequencies of some of the modes observed in the present work agree well with the literature reported values obtained from FIR and Raman studies, for others no such information is available. The present study demonstrates the usefulness of positron lifetime spectroscopy for investigating molecular motions. Introduction Molecular motions in condensed matter have been widely investigated with conventional techniques like Infrared and Raman spectroscopy. These techniques have also been employed to elucidate the molecular structure and the nature of intermolecular interactions in such materials. Recently positron lifetime spectroscopy (PLS) has also been used [1-3] to study some molecular motions in liquid crystal forming materials. This technique is based on the sensitivity of positron annihilation characteristics to the physical state of the system. On rapid cooling, a liquid may not solidify into a crystalline solid but may go to a super cooled state and then finally transform into a glassy or amorphous solid, although it is thermodynamically unstable. The formation of such a glassy state has been observed in several compounds [4-10]. The relaxation times of glassy systems are large compared to macroscopic observation times; therefore, they provide an ideal situation for studying the complex molecular dynamics. The formation of a glass forming super cooled state is believed to be a direct consequence of low degree of symmetry of the unstrained molecules [11]. Therefore, the systems composed of large asymmetric molecules have a higher chance of super cooling to a glass forming state. The typical cooling rates for such systems to undergo super cooling are conveniently low, ~ 0.5 Ks 1 . The molecules of liquid crystal forming compound n-p-ethoxybenzylidine-p-butylaniline (EBBA) are highly asymmetric and have the ability to form a disordered glassy state on being quenched from the isotropic or liquid crystalline phase. This compound has, therefore, been chosen for the present investigation. Normally, in the glassy state all molecular motions get frozen. However, the availability of free volume in such a state provides a room for the execution of some segmental motions of the Materials Science Forum Online: 2004-01-15 ISSN: 1662-9752, Vols. 445-446, pp 244-248 doi:10.4028/www.scientific.net/MSF.445-446.244